Actuators By themselves, valves cannot control a process. Manual valves require an operator to...

ActuatorsBy themselves, valves cannot control a process. Manual valves require an operator to position them to control a process variable. Valves that must be operated remotely and automatically require special devices to move them. These devices are called actuators. Actuators may be pneumatic, hydraulic, or electric solenoids or motors

1BEE4523 Chapter 3 : ActuatorNH


Actuators

1 Electrical actuator2 Pneumatic3 Hydraulic4 Fluid valve


Electrical Actuator


Electrical Actuator

2 types of electrical actuators :

• Solenoid

• Electrical Motors

Dc Motor

Ac Motor

Stepping motor


Solenoid

• Solenoid : converts an electrical

signal into mechanical motion,

usually rectilinear

• Used when a large, sudden force

must be applied to perform some

job


Solenoid

• Consist of a coil n plunger (freestanding / spring

loaded)

• Coil will have voltage or current rating in dc or ac


Solenoid


Solenoid

• Solenoid used to change the gears of a two-

position transmission

• Used SCR to activate solenoid coil

Electric Solenoid Actuators A typical electric solenoid actuator is shown in Figure 38. It consists of a coil, armature, spring, and stem.The coil is connected to an external current supply. The spring rests on the armature to force it downward. The armature moves vertically inside the coil and transmits its motion through the stem to the valve. When current flows through the coil, a magnetic field forms around the coil. The magnetic field attracts the


armature toward the center of the coil. As the armature moves upward, the spring collapses and the valve opens. When the circuit is opened and current stops flowing to the coil, the magnetic field collapses. This allows the spring to expand and shut the valve.A major advantage of solenoid actuators is their quick operation. Also, they are much easier to install


than pneumatic or hydraulic actuators. However, solenoid actuators have two disadvantages. First, they have only two positions: fully open and fully closed. Second, they don’t produce much force, so they usually only operate relatively small valves.



Figure 38 Electric Solenoid Actuator


Electrical Motors

• Electrical motors : accept electrical

input and produce a continuous rotation

as a result

• Motor styles and sizes vary as demands

for rotational speed (rpm), starting

torque, rotational torque, etc


DC Motor

• Rotation of dc motor :

produced by the

interaction of two

constant magnetic fields

• Employs permanent

magnet (PM) to form one

of the magnetic fields

• 2nd magnetic field formed

by current through coil of

wire within PM


DC Motor

• Coil of wire

(armature) : free

to rotate

• Coil is connected

to current source

through slip rings

and brushes

(commutator)

• Slip rings are split

so that current

reverses direction

as the armature

rotates


DC Motor


DC Motor

• Torque will drive

the N armature

from the N PM &

the S armature

from the S PM

• Armature rotate

counterclockwise

• Rotation will

continuous


DC Motor

• Many dc motors use electromagnet instead of

PM to provide static field

• Field coil : coil used to produce static field

• Wound field motor : dc motor to produce

static field

• Current can be provided by placing the coil in

series or parallel (shunt) or else with two

windings (compound) with the armature


Series field

• Large starting

torque

• Difficult to speed

control

• Good for starting

heavy, where

speed control is

not important


Shunt field

• Smaller starting

torque

• Good speed

control

• Good for

application

where speed is

to be controlled


Compound field

• The best

features of

both series &

shunt

• Large starting

torque

• Good speed

control


AC Motor

• Principle of ac motor : involves the

interaction between 2 magnetic

fields

• Both fields varying with the voltage

• Force between fields is a function of

rotor angle and current phase


Synchronous ac motor

• AC voltage applied to field coils (stator)

• Armature (rotor) is PM or dc electromagnet

• Rotor follows

stator

• Speed of rotation:

rpmp

fns

120


Induction ac motor

• Rotor is neither PM nor dc electromagnet

• Current induced from stator coils

• Ac field of stator

produces

magnetic field

changing through

closed loop of

rotor


Stepping Motor

• Stepping motor : rotating machine that

completes a full rotation by sequencing

through a series of discrete rotational

steps

• Rotational rate : determined by the

number of steps per revolution and the

rate at which the pulses are applied


Stepping motor

• 90° per step

• Rotor is PM driven by set of electromagnets

• Switches : transistors, SCRs or TRIACs

• Switch sequencer will direct the switches through a sequence of positions as the pulses received


Stepping motor

• Pulse will change S2 from C to D

• Poles of electromagnet reversing fields

• Pole N/S orientation is different

• Rotor repelled and attracted, moves to new position

• Next pulse will change S1 from A to B – same kind of pole reversal and rotation of rotor


Stepping motor

• Next pulse will change S2 from D to C– same kind of pole reversal and rotation of rotor

• Next pulse will change S1 from B to A– same kind of pole reversal and rotation of rotor, send back system to original position

• Continuous sequence as pulse come in


Example 1

A stepper motor has 10° per step

and must rotate at 250 rpm.

Calculate the required input pulse

rate, in pulses per second.

Electric Motor Actuators Electric motor actuators vary widely in their design and applications. Some electric motor actuators are designed to operate in only two positions (fully open or fully closed). Other electric motors can be positioned between the two positions. A typical electric motor actuator is shown in Figure 39. Its major parts include an electric motor, clutch and gear box assembly, manual hand wheel, and stem connected to a valve.


The motor moves the stem through the gear assembly. The motor reverses its rotation to either open or close the valve. The clutch and clutch lever disconnects the electric motor from the gear assembly and allows the valve to be operated manually with the hand wheel. Most electric motor actuators are equipped with limit switches, torque limiters, or both. Limit switches de-energize the electric motor when the valve has reached a specific position. Torque Rev. 0 Page 59 IC-07


Limiters de-energize the electric motor when the amount of turning force has reached a specified value. The turning force normally is greatest when the valve reaches the fully open or fully closed position. This feature can also prevent damage to the actuator or valve if the valve binds in an intermediate position.



Figure 39 Electric Motor Actuator


Pneumatic


Pneumatic

• Pneumatic : based on concept of pressure

as force per unit area

• Net force acts on diaphragm ;

NforceF

mareadiaphragmA

Padifferencepressurepp

ppAF

:

:

:2

21

21


Direct Pneumatic

• Condition in low

signal-pressure state

• Spring, S maintains

diaphragm and

connected control

shaft in position

• Pressure on opposite

side is maintained at

atmospheric

pressure by open

hole, H


Direct Pneumatic

• Increasing control

pressure applies

force on diaphragm

• Forcing diaphragm

and connected shaft

down against spring

force

• Maximum control

pressure and travel

of shaft


Pneumatic

• Shaft position is linearly related to applied control pressure

• Shaft position;

mNconstantspringk

mareadiaphragmA

Papressuregaugeappliedp

mtravelshaftx

pk

Ax

/:

:

:

:

2


Reverse Pneumatic

• Moves shaft in

opposite sense

from direct

actuator

• Obeys same

operating principle

• Shaft is pulled by

application of

control pressure


Example 2

A force of 400 N must be applied to open

valve. Determine the diaphragm area if a

control gauge pressure of 70 kPa (~10

psi) must provide this force.

Pneumatic Actuators A simplified diagram of a pneumatic actuator is shown in figure 35. It operates by a combination of force created by air and spring force. The actuator positions a control valve by transmitting its motion through the stem. A rubber diaphragm separates the actuator housing into two air chambers. The upper chamber receives supply air through an opening in the top of the housing.


The bottom chamber contains a spring that forces the diaphragm against mechanical stops in the upper chamber. Finally, a local indicator is connected to the stem to indicate the position of the valve. The position of the valve is controlled by varying supply air pressure in the upper chamber. This results in a varying force on the top of the diaphragm. Initially, with no supply air, the spring forces the diaphragm upward against the mechanical stops and holds the valve fully open. As supply air pressure is increased


from zero, its force on top of the diaphragm begins to overcome the opposing force of the spring. This causes the diaphragm to move downward and the control valve to close.With increasing supply air pressure, the diaphragm will continue to move downward and compress the spring until the control valve is fully closed. Conversely, if supply air pressure is decreased, the spring will begin to force the diaphragm upward and open the control valve. Additionally, if supply pressure is held constant at


some value between zero and maximum, the valve will position at an intermediate position. Therefore, the valve can be positioned anywhere between fully open and fully closed in response to changes in supply air pressure. A positioner is a device that regulates the supply air pressure to a pneumatic actuator. It does this by comparing the actuator’s demanded position with the control valve’s actual position. The demanded position is transmitted by a pneumatic or electrical control signal from a controller to the


positioner. The pneumatic actuator in Figure 35 is shown in Figure 36 with a controller and positioner added. The controller generates an output signal that represents the demanded position. This signal is sent to the positioner. Externally, the positioner consists of an input connection for the control signal, a supply air input connection, a supply air output connection, a supply air vent connection, and a feedback linkage. Internally, it contains an intricate network of electrical transducers, air lines, valves, linkages, and


necessary adjustments. Other positioners may also provide controls for local valve positioning and gauges to indicate supply air pressure and control air pressure (for pneumatic controllers). From an operator’s viewpoint, a description of complex internal workings of a positioner is not needed. Therefore, this discussion will be limited to inputs to and outputs from the positioner. In Figure 36, the controller responds to a deviation of a controlled variable from set point and varies the control output signal accordingly to


correct the deviation. The control output signal is sent to the positioner, which responds by increasing or decreasing the supply air to the actuator. Positioning of the actuator and control valve is fed back to the positioner through the feedback linkage. When the valve has reached the position demanded by the controller, the positioner stops the change in supply air pressure and holds the valve at the new position. This, in turn, corrects the controlled variable’s deviation from set point.


For example, as the control signal increases, a valve inside the positioner admits more supply air to the actuator. As a result, the control valve moves downward. The linkage transmits the valve position information back to the positioner. This forms a small internal feedback loop for the actuator. When the valve reaches the position that correlates to the control signal, the linkage stops supply air flow to the actuator. This causes the actuator to stop. On the other hand, if the control signal decreases, another


This causes the valve to move upward and open. When the valve has opened to the proper position, the inside the positioner opens and allows the supply air pressure to decrease by venting the supply air.positioner stops venting air from the actuator and stops movement of the control valve. An important safety feature is provided by the spring in an actuator. It can be designed to position a control valve in a safe position if a loss of supply air occurs. On a loss of supply air, the actuator in Figure 36 will


fail open. This type of arrangement is referred to as "air-to-close, spring-to-open" or simply "fail-open." Some valves fail in the closed position. This type of actuator is referred to as "air-to-open, spring-to-close" or "fail-closed." This "fail-safe" concept is an important consideration in nuclear facility design



Figure 36 Pneumatic Actuator with Controller and Positioner


Hydraulic


Hydraulic

• Hydraulic : used when large forces are

required

• Hydraulic pressure ;

21

1

11

:

:

:

/

mareapistonforcingA

NforcepistonappliedF

Papressurehydraulicp

AFp

H

H


Hydraulic

• Basic idea same as

pneumatic

• Except

incompressible fluid

used to provide

pressure

• Pressure will be very

large by adjusting

area of forcing piston


Hydraulic

allows the lifting of a heavy load with a small force


Hydraulic

• Pressure is transferred equally throughout the liquid

• Force on working piston;

• Working force in terms of applied force;

22

2

:

:

mareapistonworkingA

NpistonworkingonforceF

ApF

w

Hw

11

2 FA

AFw


Automobile Hydraulic Lift

• A hydraulic lift for automobiles is an example of a force multiplied by hydraulic press, based on Pascal's principle. The fluid in the small cylinder must be moved much further than the distance the car is lifted.


Pascal’s Principle


Example 3

Determine:

(a)The working force resulting from 200 N

applied to a 1 cm radius forcing piston if

the working piston has a radius of 6 cm

(b)The hydraulic pressure


Example 4

If the lift cylinder were 25 cm in diameter

and the small cylinder were 1.25 cm in

diameter, determine the force on the fluid

in the small cylinder to lift a 6000 N car.

Hydraulic Actuators Pneumatic actuators are normally used to control processes requiring quick and accurate response, as they do not require a large amount of motive force. However, when a large amount of force is required to operate a valve (for example, the main steam system valves), hydraulic actuators are normally used. Although hydraulic actuators come in many designs, piston types are most common.


A typical piston-type hydraulic actuator is shown in Figure 37. It consists of a cylinder, piston, spring, hydraulic supply and returns line, and stem. The piston slides vertically inside the cylinder and separates the cylinder into two chambers. The upper chamber contains the spring and the lower chamber contains hydraulic oil. The hydraulic supply and return line is connected to the lower chamber and allows hydraulic fluid to flow to and from the lower chamber of the actuator. The stem transmits the motion of the piston to a valve


Initially, with no hydraulic fluid pressure, the spring force holds the valve in the closed position. As fluid enters the lower chamber, pressure in the chamber increases. This pressure results in a force on the bottom of the piston opposite to the force caused by the spring. When the hydraulic force is greater than the spring force, the piston begins to move upward, the spring compresses, and the valve begins to open. As the hydraulic pressure increases, the valve continues to open. Conversely, as hydraulic


oil is drained from the cylinder, the hydraulic force becomes less than the spring force, the piston moves downward, and the valve closes. By regulating amount of oil supplied or drained from the actuator, the valve can be positioned between fully open and fully closedThe principles of operation of a hydraulic actuator are like those of the pneumatic actuator. Each uses some motive force to overcome spring force to move the valve. Also, hydraulic actuators can be designed to fail-open or fail-closed to provide a fail-safe feature.



Figure 37 Hydraulic Actuator


Fluid valve


Control-valve principles

• Flow rate in process control : volume per unit time

• If a given fluid is delivered through a pipe, the volume flow rate ;

smvelocityflowv

mareapipeA

smrateflowQ

AvQ

/:

:

/:2

3



• To regulate flow rate of fluids through pipes

• Placing variable-size restriction in flow path

• Stem and plug move up and down

• Opening size between plug and seat changes, thus changing flow rate



• There will be a drop in pressure and flow rate varies with square root of pressure drop

• The volume flow rate ;

Padifferencepressureppp

PasmconstantalityproportionK

smrateflowQ

pKQ

:

//:

/:

12

2/13

3


Control-valve types

• Control-valve characteristic :

different types of control valves

depends on relationship between

valve stem position and flow rate

through valve


Control-valve types

• Control

valve using

pneumatic

actuator

• To drive

stem and

open or

close valve


Control-valve types

• Types determined by shape of plug and

seat

• Stem and plug move with respect to seat

• Shape of plug determines the amount of

actual opening valve


Control-valve types

1. Quick opening

• Used predominantly for full ON/full

OFF control applications

• Small motion of valve stem, max

possible flow rate

• Allow 90% max flow rate, 30% travel

of stem


Control-valve types

2. Linear

• Flow rate varies linearly with stem position

• Ideal situation, valve determines pressure drop;

mpositionstemmaximumS

mpositionstemS

smrateflowmaximumQ

smrateflowQ

S

S

Q

Q

:

:

/:

/:

max

3max

3

maxmax


Control-valve types

3. Equal percentage

• Percentage change in stem position produces equivalent change in flow-equal percentage

• Not shut off the flow completely in its limit of stem travel

rateflowminimumQ

rateflowmaximumQ

Q

QR

:

:

min

max

min

max

max/max

SSRQQ

• Rangeability;

• Flow rate;


Control-valve types

• Three

types of

control

valves

open

differently

as function

of stem

position


Control-valve sizing

• Correction factor (valve flow coefficient), Cv : allow

selection of proper size of valve to accommodate flow rate

• Measured as the number of U.S. gallons of water per minute

that flow through full open valve with pressure differential 1 lb

per square inch

• Liquid flow rate (in U.S. gallons per minute);

gravityspecificliquidS

psivalveacrosspressurep

S

pCQ

G

Gv

:

:


Control-valve sizing

• Control-valve

flow

coefficients for

different size

valves

Valve size (inches) Cv

¼ 0.3

½ 3

1 14

1½ 35

2 55

3 108

4 174

6 400

8 725


Example 5

Alcohol is pumped through a pipe of

10 cm diameter at 2 m/s flow

velocity. Calculate the volume flow

rate


Example 6

A pressure difference of 1.1 psi occurs

across a constriction in a 5 cm diameter

pipe. The constriction constant is 0.009

m3/s/kPa1/2. Determine:

(a) The flow rate in m3/s

(b) The flow velocity in m/s


Example 7

An equal percentage valve has a

maximum flow of 50 cm3/s and a

minimum of 2 cm3/s. If the full

travel is 3 cm, calculate the flow at

a 1cm opening.


Example 8

Calculate:

(a) The proper Cv for a valve that must

allow 150 gal of ethyl alcohol per

minute with a specific gravity of 0.8

at maximum pressure of 50 psi

(b) The required valve size

Summary The important information in this chapter is summarized below.Valve Actuator Summary Pneumatic actuators utilize combined air and spring forces for quick accurate responses for almost any size valve with valve position ranging from 0-100%.


Hydraulic actuators use fluid displacement to move a piston in a cylinder positioning the valve as needed for 0-100% fluid flow. This type actuator is incorporated when a large amount of force is necessary to operate the valve.


Solenoid actuators are used on small valves and employ an electromagnet to move the stem which allows the valve to either be fully open or fully closed. Equipped with limit switches and/or torque limiters, the electric motor actuator has the capability of 0-100% control and has not only a motor but also a manual hand wheel, and a clutch and gearbox assembly. End of text.


Actuators By themselves, valves cannot control a process. Manual valves require an operator to...

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